There is an information transmission method in which information terminals that fail to establish a network connection, to each other communicate, by temporarily storing an exchange notification information with each other in the information terminals, and sending the information when wireless communication to and from another information terminal becomes available. This method is commonly called delay tolerant networking (DTN), and the following notification information is called a bundle, a message, or the like. DTN is also an abbreviation of Disruption Tolerant Networking.
The concrete flow of information transmission is as follows in general. A given information terminal determines an arbitrary information terminal as the destination of notification information. But, the given information terminal recognizes that there are no other terminals with which the given information terminal can connect in its communication area at present. In this case, the given information terminal temporarily saves the notification information inside instead of transmitting. When other terminal with which the given information terminal can communicate subsequently enters the communication area, as a result of the moving of the given information terminal itself or the moving of the other terminal, the given information terminal establishes connection to the other terminal within the range of communication to send the temporarily saved notification information.
The information terminal that receives the notification information transmits the notification information, in the case where connection to other terminals with which the information terminal is communicable is established. And the information terminal temporarily saves the notification information inside instead of transmitting, in the case where there are no other communication terminals within its communication range. This operation is repeated by many terminals until the notification information reaches the arbitrary information terminal determined as the final destination. This information transmission method is dependent on chance and therefore not particularly efficient, but is an effective way of communication in a situation where a densely-connected network is not available.
Ad hoc networking is a communication technology similar to delay tolerant communication. The two resemble each other in that notification information is transmitted hop by hop between information terminals. On the other hand, ad hoc networking is a technology that is used only when a network path from a transmission end to a destination end is established (as terminal themselves or as the overall system), and does not transmit information by chance with a transmission route indeterminate at the time of initial transmission of the information as in delay tolerant communication. In other words, the transmission route in ad hoc networking is established as a wireless communication system at the time information is issued.
The premise of communication by ad hoc networking, which is called so because a wireless network is built dynamically, is nonetheless to build a network from the transmission end to the destination. Accordingly, ad hoc networking does not work in a situation where the connection path is indeterminate, such as one where information terminals are dispersed over a wide area. Delay tolerant communication is therefore important as means of information transmission under this condition.
Temporarily saving notification information to be transmitted and transmitting the saved information once communication becomes possible is the basic mode of delay tolerant communication. The feature of temporarily saving notification information to be transmitted and then communicating the saved information to another information terminal can be applied to obtain an information transmission method described below. This information transmission method is sometimes called store-and-forward transfer or ferry transfer.
When there are several stationary information sources (or information source groups) among which communication is not established, information terminals having a delay tolerant communication function (DTN terminals) are prepared and circuited around the information sources on set paths. The information terminals approach each the information source to obtain and keep notification information from the information sources, and to transmit notification information obtained near another information source to this information source. The information terminal with the delay tolerant communication function, which moves from location to location, may be called a message ferry (or simply a ferry).
DTN technology and a ferry are disclosed in Patent Document 1. Disclosed in this document is a mechanism of preventing an unnecessary exchange of notification information, by providing a scheme for storing and keeping in a DTN terminal as communication path information (communication connection history information which records terminals within the range of communication and terminals having a chance of communication) another DTN terminal to which the DTN terminal in question is currently connected and other DTN terminals to which the DTN terminal in question was connected in the past, and a scheme for exchanging the communication path information with another DTN terminal.
By supporting the relay transmission of notification information in conformity with the concept of delay tolerant communication in the manner described above, information can be transmitted between information sources that fail to establish a network connection to each other. This method is an application of the function of temporarily saving notification information to be transmitted, which is a part of the delay tolerant communication function. One of indices that are given importance in this communication method is a delay in information transmission, and how to reduce the transmission delay as much as possible is being studied as disclosed in Non Patent Document 1. An approach to solve a similar problem is disclosed in Non Patent Document 2.